Polypeptides derived from RNA polymerases and use thereof

ABSTRACT

Mutant RNA polymerases of phagic origin in which the peptide chain is modified by substitution, deletion or addition of at least one amino acid, the modification having the effect of reducing the sensitivity of the RNA polymerases to the initial transcription sequence of the DNA sequence coding for the RNA, for a method for production of the RNA, or proteins coded by the RNA, from given nucleotide sequences, comprising a sequence of DNA coding for the RNA, the transcription of which is placed under the control of a promoter recognised by wild-type RNA polymerases and the mutant RNA polymerases as above. The method has a higher yield of RNA than the yield obtained when using the wild-type RNA polymerases in the presence of the same non-consensual ITS as that found in the sequence of DNA coding for the RNA.

A subject of the present invention is polypeptides derived from RNA polymerases, also designated mutated RNA polymerases, as well as their uses, in particular in the field of in vivo or in vitro preparation of RNA of interest.

When obtaining large quantities of DNA molecules of perfectly defined sequence, high purity and small size (10 to 100 nt) chemical synthesis is at present the most economical method. There are now in the market a very large number of enterprises which carry out these syntheses on request. These same companies also offer the chemical synthesis of RNA but the technology is such that the cost, which is at least ten times higher than for DNA, is at present prohibitive.

Enzyme synthesis is therefore favoured, using an RNA polymerase coupled to a DNA matrix, chemically synthesized or of plasmid origin, which comprises the sequence to be transcribed downstream of the promoter sequence of the polymerase used. The necessary elements (polymerases, rNTP, cloning vectors) are available on the market in the form of optimized kits.

Bacteriophage T7 RNA polymerase is very widely used for carrying out transcription reactions in vitro with the aim of synthesizing large quantities (up to a milligram) of RNA from matrices of recombinant DNA. These RNAs synthesized in vitro are necessary for a number of uses in Biology, the Biotechnologies and Pharmacy; the following can be mentioned inter alia:

-   -   translation in vitro.     -   the study of RNA maturation.     -   the effect of anti-sense RNA on gene expression.     -   RNA-protein interactions.     -   the synthesis of homologues of small cell RNAs (tRNA, rRNA).     -   the production of ribozymes.

More generally these RNAs are also used as molecular probes and structural study substrates.

A few T7 polymerase mutants have already been described. These are mutated RNA polymerases derived from the wild-type T7 RNA polymerase, and comprising one of the following mutations:

-   -   replacement of the lysine (K) in position 222 by glutamic acid;         this RNA polymerase thus mutated has the property of recognizing         a promoter altered by mutation, and is described in U.S. Pat.         No. 5,385,834,     -   replacement of the tyrosine (Y) in position 639 by a         phenylalanine (F); this RNA polymerase thus mutated has the         property of incorporating deoxyribonucleotides instead of         ribonucleotides, thus allowing the synthesis of DNA instead of         RNA, and is described in U.S. Pat. No. 6,107,037,     -   replacement of the isoleucine (I) in position 810 by a serine         (S); this RNA polymerase thus mutated has the property of being         slower than the wild-type RNA polymerase, and is described in         Bonner et al., The Journal of Biochemical Chemistry, 269, pp.         25120-25128 (1994).

The problem to be resolved by the present invention, is linked with the fact that the activity on a given DNA matrix of the T7 RNA polymerase (and moreover of all the other known RNA polymerases) is strongly dependent on the nature of the first 6-12 nucleotides transcribed (Initial Transcribed Sequence, or ITS) of the nucleotide sequence coding for a given RNA (Milligan et al., 1987). If the ITS differs too much from the consensus sequence 5′GGGAGA . . . then the polymerase frequently aborts and a large quantity of the ribonucleoside triphosphates is consumed in order to synthesize small abortive RNAs to the detriment of the desired large RNAs.

Therefore, the experimenter is generally compelled, in order to obtain an effective transcription, to modify the sequence that is to be transcribed, in order to add to it a favourable ITS (also referred to as consensus or consensual ITS). In a number of cases, this constraint is very annoying, even unacceptable.

A potential method for obtaining any non-consensual 5′ end RNA uses an after treatment of an RNA comprising the desired sequence downstream of a consensus ITS; the latter (target) RNA can therefore be produced in abundance. Moreover it is necessary to chemically synthesize a chimeric DNA-RNA oligonucleotide the DNA sequence of which is complementary to the 3′ end of the RNA to be eliminated, then the hybrid that can form the oligo with the target transcript is digested by RNase H. This complex and onerous method is justified if the desired RNA is of a very large size and if its 5′ end must be rigorously defined. (Li et al. 1999, Lapham et al. 1997).

The addition, to the reaction medium, of synthetic polyamines can increase the production of RNA by, it appears, reducing the number of abortive cycles; however determination of the optimum experimental conditions (choice of polyamine, concentration of use) requires case-by-case research. Moreover it should be noted that the effect of these polyamines is weak, even non-existent, when the matrix is of plasmid origin (double strand) (Frugier et al. 1994).

These two methods use reagents which are not in common use (specific polyamines or oligonucleotides) and are unsuitable for routine use. It is evident that the possibility of having, at the start, a polymerase which by nature aborts less would be a much simpler way of resolving the difficulty.

The invention results from the demonstration by the Inventors of the fact that modifying the peptide sequence of the RNA polymerases could reduce the sensitivity of the RNA polymerases thus modified to the nature of the ITS of the nucleotide sequences to be transcribed.

The invention aims to provide polypeptides derived from wild-type RNA polymerases usually used in the production of RNA of interest, which are distinctly less sensitive to the nature of the non-consensual ITS than said wild-type RNA polymerases.

The invention also aims to provide new processes for producing RNA of interest with yields greater than those of the processes using wild-type RNA polymerases when the latter are sensitive to the ITS of the nucleotide sequence coding for said RNA of interest.

A subject of the invention is the use of mutated RNA polymerases of phage origin, namely of RNA polymerases originating from phages, the peptide chain of which is modified, with respect to the wild-type RNA polymerases from which they derive, by substitution, or deletion, or addition of at least one amino acid, this modification having the effect of reducing the sensitivity of said RNA polymerases to the ITS contained in the DNA sequence coding for an RNA of interest, for the implementation of a process for producing said RNA of interest, or of proteins coded by this RNA of interest, starting with determined nucleotide sequences comprising a DNA sequence coding for said RNA of interest and the transcription of which is placed under the control of a promoter recognized by the abovementioned wild-type RNA polymerases and mutated RNA polymerases, said process having a production yield of said RNA greater than the yield obtained in the case of use of the wild-type RNA polymerases (in the presence of the same non-consensual ITS as that contained in the DNA sequence coding for said RNA of interest).

The activity of said RNA polymerases thus mutated within the transcription of the DNA sequence coding for said RNA of interest is distinctly less affected by the nature of the nucleotides constituting the non-consensual ITS of this DNA sequence, which is not that of the wild-type RNA polymerases from which they derive, which allows these mutated RNA polymerases to be up to approximately 40 times more active than the wild-type polymerases, and therefore, in the abovementioned process of the present invention, to be characterized by a production yield of said RNA greater than the yield obtained in the case of use of the wild-type RNA polymerases in the presence of this same non-consensual ITS.

According to another particularly advantageous aspect of the invention, the mutated RNA polymerases of the invention make it possible to obtain RNA of interest with a practically identical yield, whatever the ITS present in the sequence coding for said RNA of interest.

Advantageously, the use of the abovementioned mutated RNA polymerases allows the implementation of a process for producing RNA of interest, the yield of which is up to approximately ten times greater than the yield obtained in the case of use of the wild-type RNA polymerases in the presence of a non-consensual ITS.

The RNA of interest capable of being produced in greater quantity within the implementation of a process according to the invention using the abovementioned mutated RNA polymerases, are natural or chimeric RNAs, said RNAs if appropriate comprising one or more non-canonical nucleoside monophosphates (namely for example a deoxyribose instead of a ribose, said sugar itself being able optionally to carry an analogue of one of the natural nucleic bases if this analogue is recognized as such by the polymerase). In the latter case, a subject of the present invention is also the use of the above-mentioned process using said above-mentioned RNA polymerases, in the implementation of a method for determining the sequence of a nucleic acid molecule.

The mutated RNA polymerases used are advantageously those deriving from wild-type phage monomeric polymerases, in particular those originating from monomeric RNA polymerases of bacteriophages such as T7, T3, K11, SP6, respectively described in particular in the article by W. T. McAllister and C. A. Raskin, Molecular Microbiology (1993), 10(1), 1-6.

Preferably the abovementioned mutated RNA polymerases are those deriving from the wild-type RNA polymerases at least one of the amino acids of which, situated between positions 1 and approximately 410, in particular approximately between positions 90 and 320, more particularly between positions 115 and 300, is modified by substitution or deletion.

Advantageously, the mutated RNA polymerases used are those comprising a leucine in position 266, substituted for the proline situated in position 266 in the wild-type T7 RNA polymerase, or the proline situated in homologous position in the wild-type RNA polymerases of bacteriophages, such as the proline situated in positions 267 in T3, 289 in K11, and 239 in SP6.

A more particular subject of the invention is the abovementioned use of any mutated RNA polymerase as defined above, the proline of which, defined above, and/or at least one of the amino acids situated in the vicinity of the abovementioned proline, namely an amino acid situated at a distance less than or equal to approximately 10 angströms from the proline in question, when said RNA polymerase is considered in its three-dimensional structure (as described in Cheetham, G. M. & Steitz, T. A. (1999), Science 286, 2305-2309; Cheetham, G. M. et al., Nature 399, 80-83; Sousa, R. et al., Nature 364, 593-599), is modified by substitution or deletion.

The invention also relates to the abovementioned use of the particularly preferred mutated RNA polymerases chosen from the following:

-   -   those derived from the wild-type T7 RNA polymerase, and         comprising at least one of the following mutations:         -   replacement of the isoleucine (I) in position 117 by a             valine (V),         -   replacement of the isoleucine (I) in position 119 by a             valine (V),         -   replacement of the valine (V) in position 134 by an alanine             (A),         -   replacement of the aspartic acid (D) in position 147 by             asparagine (N),         -   replacement of the histidine (H) in position 230 by an             arginine (R),         -   replacement of the proline (P) in position 266 by a leucine             (L),         -   replacement of the arginine (R) in position 291 by a             cysteine (C),     -   those derived from the wild-type T3 RNA polymerase, and         comprising at least one of the following mutations:         -   replacement of the aspartic acid (D) in position 148 by             asparagine (N),         -   replacement of the proline (P) in position 267 by a leucine             (L),         -   replacement of the arginine (R) in position 292 by a             cysteine (C),     -   those derived from the wild-type K11 RNA polymerase, and         comprising at least one of the following mutations:         -   replacement of the aspartic acid (D) in position 167 by             asparagine (N),         -   replacement of the proline (P) in position 289 by a leucine             (L),         -   replacement of the arginine (R) in position 314 by a             cysteine (C),     -   those derived from the wild-type SP6 RNA polymerase, and         comprising at least one of the following mutations:         -   replacement of the aspartic acid (D) in position 117 by             asparagine (N),         -   replacement of the proline (P) in position 239 by a leucine             (L).

A more particular subject of the invention is the abovementioned use of the following mutated RNA polymerases:

-   -   the mutated T7 RNA polymerase represented by SEQ ID NO: 2,         comprising a leucine in position 266 substituted for the         proline,     -   the mutated T7 RNA polymerase represented by SEQ ID NO: 4,         comprising a valine in position 117 substituted for the         isoleucine, and an alanine in position 134 substituted for the         valine,     -   the mutated T7 RNA polymerase represented by SEQ ID NO: 6,         comprising a valine in position 119 substituted for the         isoleucine, and an asparagine in position 147 substituted for         the aspartic acid,     -   the mutated T7 RNA polymerase represented by SEQ ID NO: 8,         comprising an arginine in position 230 substituted for the         histidine, and a cysteine in position 291 substituted for the         arginine,     -   the mutated T7 RNA polymerase represented by SEQ ID NO: 10,         comprising a leucine in position 266 substituted for the         proline, and a phenylalanine in position 639 substituted for the         tyrosine,     -   the mutated T7 RNA polymerase represented by SEQ ID NO: 12,         comprising an asparagine in position 810 substituted for the         isoleucine,     -   the mutated T7 RNA polymerase represented by SEQ ID NO: 14,         comprising a leucine in position 266 substituted for the         proline, and an asparagine in position 810 substituted for the         isoleucine,     -   the mutated T7 RNA polymerase represented by SEQ ID NO: 16,         comprising a valine in position 119 substituted for the         isoleucine, an asparagine in position 147 substituted for the         aspartic acid, and an asparagine in position 810 substituted for         the isoleucine.

A subject of the invention is also the use described above of abovementioned mutated RNA polymerases comprising as well as the modification(s) of amino acid(s) defined above, the modification, by substitution, or deletion, or addition, of at least one other amino acid involved in a mechanism other than that of sensitivity to the ITS within the scope of the transcription of DNA sequences, such as a modification of an amino acid making it possible for the RNA polymerase thus modified to incorporate non-canonical nucleoside triphosphates (for example dNTP) in an RNA chain, and therefore making it possible to synthesize DNA, and/or a modification of an amino acid making it possible to obtain a slower RNA polymerase, and therefore, in association with a mutation of the invention, making it possible to improve the synthesis yields of proteins of interest.

A more particular subject of the invention is therefore the abovementioned use of mutated RNA polymerases as defined above, comprising as well as the mutation(s) defined above, a mutation corresponding to the substitution of the tyrosine in position 639 in T7, or in analogous position in the other polymerases defined above, by a phenylalanine, and/or a mutation corresponding to the substitution of the isoleucine in position 810 in T7, or in analogous position in the other polymerases defined above, by a serine, or by an asparagine.

Therefore, the invention also relates more particularly to the use of the abovementioned mutated T7 RNA polymerase represented by SEQ ID NO: 10.

A subject of the invention is also:

-   -   the mutated RNA polymerases derived from the wild-type T7 RNA         polymerase comprising at least one of the abovementioned         mutations in position 117, 119, 134, 147, 230, 266, or 291, and         optionally at least one of the following additional mutations:         -   replacement of the tyrosine (Y) in position 639 by a             phenylalanine (F),         -   replacement of the isoleucine (I) in position 810 by an             asparagine (N),     -   the mutated RNA polymerases derived from the wild-type T3 RNA         polymerase comprising at least one of the abovementioned         mutations in position 148, 267, or 292, and optionally at least         one of the following additional mutations:         -   replacement of the tyrosine (Y) in position 640 by a             phenylalanine (F),         -   replacement of the isoleucine (I) in position 811 by an             asparagine (N),     -   the mutated RNA polymerases derived from the wild-type K11 RNA         polymerase comprising at least one of the abovementioned         mutations in position 167, 289, or 314, and optionally at least         one of the following additional mutations:         -   replacement of the tyrosine (Y) in position 662 by a             phenylalanine (F),         -   replacement of the isoleucine (I) in position 833 by an             asparagine (N),     -   the mutated RNA polymerases derived from the wild-type SP6 RNA         polymerase comprising at least one of the abovementioned         mutations in position 117 or 239, and optionally at least one of         the following additional mutations:         -   replacement of the tyrosine (Y) in position 631 by a             phenylalanine (F),         -   replacement of the isoleucine (I) in position 804 by an             asparagine (N).

The invention also relates to any process for preparing mutated RNA polymerases of phage origin as defined above, namely of RNA polymerases, the determined nucleotide sequence transcription activity of which, comprising a DNA sequence coding for an RNA of interest and the transcription of which is placed under the control of a promoter recognized by the mutated RNA polymerases and by the wild-type RNA polymerases of phage origin from which they originate, is greater than the transcription activity of this determined sequence by the wild-type RNA polymerases, in particular preparation of mutated RNA polymerases being up to approximately 40 times more active than said wild-type RNA polymerases within the scope of the implementation of processes for producing RNA of interest starting with said determined nucleotide sequence, said process comprising:

-   -   a stage of modification of the peptide chain of the wild-type         RNA polymerases of phage origin by substitution, or deletion or         addition of at least one codon of the gene coding for said         wild-type RNA polymerases, and transformation of appropriate         cells, such as E. coli, with vectors containing the gene thus         modified,     -   detection of the abovementioned cells producing mutated RNA         polymerases for which the production yield of a particular         marker within appropriate cells, such as E. coli, in particular         within the abovementioned cells, such as a marker of resistance         to an antibiotic, or a chromogenic marker, coded by a nucleotide         sequence inserted downstream of the promoter recognized by the         abovementioned RNA polymerases, this promoter and the sequence         coding for the marker being separated by an ITS, the nature of         approximately the first 6 to 12 nucleotides of which is known to         affect the activity of the wild-type RNA polymerases, is greater         than the production yield of this same marker obtained by use of         the wild-type RNA polymerases under the same conditions,     -   purification of the abovementioned mutated RNA polymerases from         the cells detected in the preceding stage.

A more particular subject of the invention is the abovementioned use of mutated RNA polymerases of phage origin as obtained by implementation of the preparation process defined above.

The invention also relates to mutated RNA polymerases of phage origin as obtained by implementation of a preparation process defined above.

A more particular subject of the invention is the mutated RNA polymerases of phage origin as obtained by implementation of a preparation process defined above, originating from the modification of wild-type phage monomeric polymerases, in particular originating from monomeric RNA polymerases of bacteriophages such as T7, T3, K11, SP6.

A subject of the invention is also the abovementioned mutated RNA polymerases of phage origin deriving from the RNA polymerases of wild-type phage origin, at least one of the amino acids of which, situated between positions 1 and approximately 410, in particular approximately between positions 90 and 320, more particularly between positions 115 and 300, is modified by substitution or deletion, to the exclusion of the mutated RNA polymerase derived from the wild-type T7 RNA polymerase, and comprising the following mutation:

-   -   replacement of the lysine (K) in position 222 by glutamic acid.

A more particular subject of the invention is the abovementioned mutated RNA polymerases comprising a leucine in position 266, substituted for the proline in position 266 in the wild-type T7 RNA polymerase, or in the homologous positions in the wild-type bacteriophage RNA polymerases, such as positions 267 in T3, 289 in K11, and 239 in SP6.

A more particular subject of the invention is also any mutated RNA polymerase as defined above, of which the proline defined above, and/or at least one of the amino acids situated in the vicinity of the abovementioned proline, namely an amino acid situated at a distance less than or equal to approximately 10 angströms from the proline in question, when said RNA polymerase is considered in its three-dimensional structure, is modified by substitution or deletion.

The invention more particularly relates to the mutated RNA polymerases derived from the wild-type RNA polymerases such as T7, T3, K11 or SP6, and chosen from the following:

-   -   those derived from the wild-type T7 RNA polymerase, and         comprising at least one of the following mutations:         -   replacement of the isoleucine (I) in position 117 by a             valine (V),         -   replacement of the isoleucine (I) in position 119 by a             valine (V),         -   replacement of the valine (V) in position 134 by an alanine             (A),         -   replacement of the aspartic acid (D) in position 147 by             asparagine (N),         -   replacement of the histidine (H) in position 230 by an             arginine (R),         -   replacement of the proline (P) in position 266 by a leucine             (L),         -   replacement of the arginine (R) in position 291 by a             cysteine (C),     -   said mutated RNA polymerases optionally comprising at least one         of the following additional mutations:         -   replacement of the tyrosine (Y) in position 639 by a             phenylalanine (F),         -   replacement of the isoleucine (I) in position 811 by an             asparagine (N),     -   those derived from the wild-type T3 RNA polymerase, and         comprising at least one of the following mutations:         -   replacement of the aspartic acid (D) in position 148 by             asparagine (N),         -   replacement of the proline (P) in position 267 by a leucine             (L),         -   replacement of the arginine (R) in position 292 by a             cysteine (C),     -   said mutated RNA polymerases optionally comprising at least one         of the following additional mutations:         -   replacement of the tyrosine (Y) in position 640 by a             phenylalanine (F),         -   replacement of the isoleucine (I) in position 811 by an             asparagine (N),     -   those derived from the wild-type K11 RNA polymerase, and         comprising at least one of the following mutations:         -   replacement of the aspartic acid (D) in position 167 by             asparagine (N),         -   replacement of the proline (P) in position 289 by a leucine             (L),         -   replacement of the arginine (R) in position 314 by a             cysteine (C),     -   said mutated RNA polymerases optionally comprising at least one         of the following additional mutations:         -   replacement of the tyrosine (Y) in position 662 by a             phenylalanine (F),         -   replacement of the isoleucine (I) in position 833 by an             asparagine (N),     -   those derived from the wild-type SP6 RNA polymerase, and         comprising at least one of the following mutations:         -   replacement of the aspartic acid (D) in position 117 by             asparagine (N),         -   replacement of the proline (P) in position 239 by a leucine             (L),     -   said mutated RNA polymerases optionally comprising at least one         of the following additional mutations:         -   replacement of the tyrosine (Y) in position 631 by a             phenylalanine (F),         -   replacement of the isoleucine (I) in position 804 by an             asparagine (N).

A more particular subject of the invention is the following mutated RNA polymerases:

-   -   the mutated T7 RNA polymerase represented by SEQ ID NO: 2,         comprising a leucine in position 266 substituted for the         proline,     -   the mutated T7 RNA polymerase represented by SEQ ID NO: 4,         comprising a valine in position 117 substituted for the         isoleucine, and an alanine in position 134 substituted for the         valine,     -   the mutated T7 RNA polymerase represented by SEQ ID NO: 6,         comprising a valine in position 119 substituted for the         isoleucine, and an asparagine in position 147 substituted for         the aspartic acid,     -   the mutated T7 RNA polymerase represented by SEQ ID NO: 8,         comprising an arginine in position 230 substituted for the         histidine, and a cysteine in position 291 substituted for the         arginine,     -   the mutated T7 RNA polymerase represented by SEQ ID NO: 10,         comprising a leucine in position 266 substituted for the         proline, and a phenylalanine in position 639 substituted for the         tyrosine,     -   the mutated T7 RNA polymerase represented by SEQ ID NO: 12,         comprising an asparagine in position 810 substituted for the         isoleucine,     -   the mutated T7 RNA polymerase represented by SEQ ID NO: 14,         comprising a leucine in position 266 substituted for the         proline, and an asparagine in position 810 substituted for the         isoleucine,     -   the mutated T7 RNA polymerase represented by SEQ ID NO: 16,         comprising a valine in position 119 substituted for the         isoleucine, an asparagine in position 147 substituted for the         aspartic acid, and an asparagine in position 810 substituted for         the isoleucine.

The invention also relates to the nucleotide sequences coding for a mutated RNA polymerase of phage origin as defined above.

Therefore a more particular subject of the invention is the nucleotide sequences SEQ ID NO: 1, 3, 5, 7, 9, 11, 13, 15, coding respectively for the abovementioned proteins SEQ ID NO: 2, 4, 6, 8, 10, 12, 14, 16, or any nucleotide sequence derived from the abovementioned nucleotide sequences by degeneration of the genetic code and retaining the property of coding for the abovementioned proteins.

A subject of the invention is also any vector, in particular any plasmid, containing a nucleotide sequence as defined above.

A subject of the invention is also any cell transformed by an abovementioned vector, said cell being chosen in particular from those of bacteria (E. coli for example), yeasts, or higher eukaryotes.

The invention also relates to any process for preparing the abovementioned mutated RNA polymerases of phage origin, comprising the culture of transformed cells defined above in an appropriate culture medium, and the purification of the RNA polymerases produced by said cells.

A subject of the invention is also any process for preparing in vitro RNA of interest, comprising the bringing together, in an appropriate medium, of at least one mutated RNA polymerase of phage origin as defined above, with the determined nucleotide sequences comprising a DNA sequence coding for said RNA of interest and the transcription of which is placed under the control of a promoter recognized by the abovementioned wild-type RNA polymerases and mutated RNA polymerases.

The invention also relates to any process for preparing in vivo RNA of interest, comprising the culture of cells as defined above, said cells producing at least one mutated RNA polymerase of phage origin according to the invention, and the genome of which has been modified in order to contain determined nucleotide sequences comprising a DNA sequence coding for said RNA of interest and the transcription of which is placed under the control of a promoter recognized by the abovementioned wild-type RNA polymerases and mutated RNA polymerases.

A subject of the invention is also any process for preparing in vivo proteins of interest comprising the culture of cells as defined above, said cells producing at least one mutated RNA polymerase of phage origin according to the invention, and the genome of which has been modified in order to contain determined nucleotide sequences comprising a DNA sequence coding for said proteins of interest and the transcription of which is placed under the control of a promoter recognized by the abovementioned wild-type RNA polymerases and mutated RNA polymerases.

Advantageously, in the case of the implementation of processes for preparing proteins of interest as described above, the mutated RNA polymerases produced by the cells, are chosen from SEQ ID NO: 12, 14 and 16.

The invention also relates to any process for preparing in vitro or in vivo RNA or proteins of interest, as described above, and comprising, moreover, a stage of addition of synthetic polyamines as described by Frugier et al. Amongst the polyamines capable of being used, the following can be mentioned: NH₂(CH₂)₃NH(CH₂)₁₀NH(CH₂)₃NH₂ NH₂(CH₂)₃NH(CH₂)₃NH(CH₂)₁₀NH(CH₂)₃NH(CH₂)₃NH₂

The invention is further illustrated using the following detailed description of the production of mutated RNA polymerases of phage origin according to the invention, and of their use for increasing the production of determined RNAs with respect to the production of these same RNAs using wild-type RNA polymerases.

Whilst the RNA polymerase of the bacteriophage T7 is normally very processive in the elongation phase, this is not the case during the transcription of the ITS: in this case, there is a high probability that the nascent transcript will be prematurely released (abortive transcription) (Martin et al., 1988). This is the number of abortive cycles that the enzyme must achieve, on average, before successfully transcribing the ITS and entering the elongation phase, which determines the frequency with which the complete transcripts are synthesized (Ikeda, 1992). This number is minimal when the ITS corresponds to the 5′ sequence of the class III genes of the bacteriophage T7 (‘consensual’ ITS: 5′ GGGAGA . . . ). However, even under these conditions, the abortive transcripts make up a large proportion (40% to 60%) of the total transcripts. This proportion increases very rapidly when the ITS is removed from the consensus, or when the RNA polymerase carries mutations which reduce its rate of elongation, increasing the time necessary for the crossing of the ITS (Bonner et al., 1994; Bonner et al., 1992). Optionally, the combination of a non-consensual ITS and a slow polymerase leads to a situation where the enzyme loops indefinitely in the abortive phase: no large transcript is then synthesized. This property can be observed not only in vitro, but also in vivo, in cells of Escherichia coli expressing T7 RNA polymerase (Lopez et al., 1997; Makarova et al., 1995). It is the latter situation that we have exploited in order to select mutants of T7 RNA polymerase, the activity of which is less sensitive to the nature of the ITS than that of the wild-type enzyme.

We placed a target gene, the expression of which is easily detectable—the lacZ gene, the product of which is β-galactosidase—preceded by a non-consensual ITS, under the control of the T7 promoter. This gene is then introduced into a bacterial cell containing moreover a plasmid coding for a T7 polymerase mutated in its catalytic site (i.e. a ‘slow’ polymerase). As indicated above, the system, blocked in abortive phase, does not produce large transcripts, and therefore no β-galactosidase. However, after random mutagenesis of the plasmid, it is possible to select bacterial clones which synthesize new mutants of the enzyme which are capable of expressing the target gene. Apart from the reverse mutants of the initial mutation of the catalytic site, we thus isolated mutations in the N-terminal part of the enzyme, which is not involved in the catalysis. When these new mutations were introduced into the wild-type enzyme, i.e. without mutations at the catalytic site, we observed that they reduced the susceptibility of the enzyme to the exact nature of the ITS. This is the point mutant P266L which is more particularly described here and forms the subject of the present invention. In fact, it makes it possible to transcribe, at usable levels, matrices on which the wild-type enzyme is practically inoperative.

DETAILED DESCRIPTION

a) Preparation, Purification and Storage

The point mutant SEQ ID NO: 2 obtained corresponds to the change in the sequence of the T7 polymerase of Proline 266 to Leucine (P266L). The gene of this mutated T7 polymerase is included in the pAR1219 plasmid under the control of an inducible promoter (pLac UV5). The protein (polymerase) is over-expressed in the BL 21 (ompF-) strain of E. coli. The protocols for extraction, purification and storage of the purified protein are identical to those of the wild-type polymerase. In order to facilitate the purification, the polymerase was labelled with six N-terminal histidines by transfer of the mutated region in place of the equivalent coding sequence of the pBH 161 plasmid. (He et al. 1977). This label in no way modifies the catalytic properties of the polymerase. It is then sufficient to pass the bacterial supernatant over a Nickel affinity column in order to obtain >90% pure protein. Storage for a long duration (2 years) at −20° C. does not alter the properties of the enzyme.

b) Catalytic Activity.

Under the standard conditions used for the wild-type polymerase, the mutant is also capable of converting all the rNTPs provided to it to RNA. However the rate at which this incorporation is carried out is approximately three times lower for the mutant, i.e. a complete transcription will require a longer period of time (e.g. 6 hours instead of 2 hours).

As referred to below, the main distinction between the wild-type polymerase and the mutant is to be found at the level of distribution as a function of the size (and therefore of the mass) of the RNAs obtained. Two types of RNA can be observed on completion of in vitro transcription: abortive RNAs and large RNAs.

We present below the in vitro transcription results obtained with the wild-type polymerase and the mutant on seven very different ITSs. The matrices are plasmid DNAs (approximately 6 kbp) linearized at restriction sites situated from 31 to 78 nt downstream of the promoter. The reactions are stopped at 10, 20 or 30 minutes depending on the case, and the marking is α³²PGTP, α³²PUTP or γ³²PGTP. The experimental conditions used are described in J. Mol. Biol. (1997) 269:41-51. The transcripts are separated on a high resolution denaturing polyacrylamide gel.

G10 (5′ GGGAGACCA . . . linearized at 33 nt) carries the consensual ITS. In fact in this case the first 30 nucleotides correspond to gene 10 sequence of the phage T7, one of those better transcribed by T7 polymerase.

GGLac (5′ GGGGAAUU. linearized at 69 nt) carries the ITS that is found in the commercial plasmid pET15b (Novagen) which comprises the binding site of the Lac repressor in order to optionally repress the T7 transcription.

Lac (5′ GGAAUUG. linearized at 30 nt) is a plasmid equivalent to the preceding one, in which the operative site is close to 2 nucleotides of the promoter.

TyrG (5′ GUCUCGG. linearized at 78 nt), Gly (5′ ACUCUUU. linearized at 71 nt), Tyr (5′ CUCUCGG. linearized at 78 nt) and Val (5′ GGUUUCG. linearized at 31 nt) are plasmids corresponding to matrices intended to synthesize in vitro tRNAs or fragments of tRNA and deemed difficult to transcribe (Frugier et al., 1994)

FIGS. 1 to 7 show the detailed results indicating the abortive RNAs observed, their percentage with respect to the total number of transcripts, as well as the equivalent figures for the long transcripts which escaped the abortive phase, in the case of use of wild-type T7 polymerase (also designated wt, and represented using black columns) and of the use of the mutated RNA polymerase SEQ ID NO: 2 (also designated P266L, and represented using grey columns) in the presence of the ITS G10 (FIG. 1), GGLac (FIG. 2), TyrG (FIG. 3), Lac (FIG. 4), Gly (FIG. 5), Tyr (FIG. 6), Val (FIG. 7). The abortive transcripts are distributed between the 2 mer and the 13 mer (numbering 2 to 13 along the x-axis). The transcripts originating from polymerases which have escaped the abortive phase are identified as long transcripts (LT along the x-axis). A logarithmic scale is used along the y-axis in order to make it possible to quantify transcripts which are not very abundant.

These results show clearly that in all cases the mutant has less difficulty than the wild-type polymerase in incorporating the fifth, sixth and seventh nucleotides. If at these positions the polymerase has and/or must incorporate a pyrimidine (U or C) the abortion rate is raised for the wild-type polymerase and the removal of the handicap by the mutant is all the greater.

The immediate consequence is summarized in FIG. 8 which includes the results referred to previously, providing figures comparing the productivities of the two polymerases for increasingly unfavourable ITSs. The values indicated correspond to the percentage of incorporation of the rNTP into large transcripts in the case of use of the wild-type T7 polymerase (wt, black columns) and the use of the mutated RNA polymerase SEQ ID NO: 2 (P266L, grey columns).

Apart from G10 and GGLac where the two polymerases give complete satisfaction, in all other cases the wild-type polymerase has a low yield, even a very low yield, comprised between 22% and 1%, whilst that of the mutant remains reasonable, being comprised between 30% and 85%.

BIBLIOGRAPHY

Bonner, G., Lafer, E. M. & Sousa, R. (1994). Characterisation of a set of T7 RNA polymerase active site mutants. J. Biol. Chem. 269, 25120-25128.

Bonner, G., Patra, D., Lafer, E. M. & Sousa, R. (1992). Mutations in T7 RNA polymerase which support the proposal for a common polymerase active site structure. EMBO J. 11, 3767-3775.

Davanloo, P., Rosenberg, A. H., Dunn, J. J. & Studier, F. W. (1984). Cloning and expression of the gene for bacteriophage T7 RNA polymerase. Proc. Natl. Acad. Sci. USA 81, 2035-2039.

Frugier, M., C., F., M. W., H., J-M., L. & R., G. (1994). Synthetic polyamines stimulate in vitro transcription by T7 RNA polymerase. Nucl. Acid Res. 22, 2784-2790.

He, B., Rong, M., Lyakhov, D., Gartenstein, H., Diaz, G., Castagna, R., McAllister, W. T. & Durbin, R. K. (1997). Rapid mutagenesis and purification of phage RNA polymerase. Protein Express Purif. 9.142-151.

Ikeda, R. A. (1992). The efficiency of promoter clearance distinguishes T7 class II and class III promoters. J. Biol. Chem. 267, 11322-11328.

Lapham J, Yu Y T, Shu M D, Steitz J A, Crothers D M.(1997) The position of site-directed cleavage of RNA using RNase H and 2′-O-methyl oligonucleotides is dependent on the enzyme source. RNA 3.950-951

LiZ, Pandit S, Deutscher M P. (1999). Maturation of 23S ribosomal RNA requires the exorbonuclease RNase T. RNA 5.139-146

Lopez, P. J., Guillerez, J., Sousa, R. & Dreyfus, M. (1997). The low processivity of T7 RNA polymerase over the initially transcribed sequence can limit productive initiation in vivo. J. Mol. Biol. 269, 41-51.

Makarova, O. V., Makarov, E. M., Sousa, R. & Dreyfus, M. (1995). Transcribing Escherichia coli genes with mutant T7 RNA polymerases: stability of lacZ mRNA inversely correlates with polymerase speed. Proc. Natl. Acad. Sci. USA 92, 12250-12254.

Martin, C. T., Muller, D. K. & Coleman, J. E. (1988). Processivity in early stages of transcription by T7 RNA polymerase. Biochemistry 27, 3966-3974.

Milligan, J. F., Groebe, D. R., Witherell, G. W. & Uhlenbeck, O. C. (1987). Oligonucleotide synthesis using T7 RNA polymerase and synthetic DNA templates. Nucleic Acids Res. 15(21), 8783-8798. 

1. A method for producing a RNA of interest, or proteins coded by this RNA of interest, comprising: synthesizing RNA with mutated RNA polymerases of phage origin, the peptide chain of which is modified, with respect to the wild-type RNA polymerases from which they originate, by substitution, or deletion, or addition of at least one amino acid, this modification having the effect of reducing the sensitivity of said RNA polymerases to the initial transcribed sequence (ITS) contained in a DNA sequence coding for an RNA of interest, starting from determined nucleotide sequences comprising a DNA sequence coding for said RNA of interest and the transcription of which is placed under the control of a promoter recognized by the abovementioned wild-type RNA polymerases and mutated RNA polymerases, said method having a production yield of said RNA greater than the yield obtained in the case of use of the wild-type RNA polymerases in the presence of the same non-consensual ITS as that contained in the DNA sequence coding for said RNA of interest, wherein said mutated RNA polymerases are selected from the group consisting of: those comprising the mutated T7 RNA polymerase represented by SEQ ID NO: 2, comprising a leucine in position 266 substituted for the proline in the wild-type T7 RNA polymerase, or of the proline situated in the homologous position in the wild-type RNA polymerases of other bacteriophages, selected from the group consisting of the proline situated in positions 267 in T3, 289 in KII, and 239 in SP6.
 2. A method for producing a RNA of interest, or proteins coded by this RNA of interest, comprising: synthesizing RNA with mutated RNA polymerases of phage origin, the peptide chain of which is modified, with respect to the wild-type RNA polymerases from which they originate, by substitution, or deletion, or addition of at least one amino acid, this modification having the effect of reducing the sensitivity of said RNA polymerases to the ITS contained in a DNA sequence coding for an RNA of interest, starting from determined nucleotide sequences comprising a DNA sequence coding for said RNA of interest and the transcription of which is placed under the control of a promoter recognized by the abovementioned wild-type RNA polymerases and mutated RNA polymerases, said method having a production yield of said RNA greater than the yield obtained in the case of use of the wild-type RNA polymerases in the presence of the same non-consensual ITS as that contained in the DNA sequence coding for said RNA of interest, wherein said mutated RNA polymerases are selected from the group consisting of: those derived from the wild-type T7 RNA polymerase, and comprising at least one of the following mutations: replacement of the isoleucine I) in position 117 by a valine (V), replacement of the isoleucine I) in position 119 by a valine (V), replacement of the valine (V) in position 134 by an alanine (A), replacement of the aspartic acid (D) in position 147 by asparagine (N), replacement of the histidine (H) in position 230 by an arginine (R), replacement of the proline (P) in position 266 by a leucine (L), replacement of the arginine (R) in position 291 by a cysteine (C), said mutated RNA polymerases optionally comprising at least one of the following additional mutations: replacement of the tyrosine (Y) in position 639 by a phenylalanine (F), replacement of the isoleucine (I) in position 810 by an asparagine (N), those derived from the wild-type T3 RNA polymerase, and comprising at least one of the following mutations: replacement of the aspartic acid (D) in position 148 by asparagine (N), replacement of the proline (P) in position 267 by a leucine (L), replacement of the arginine (R) in position 292 by a cysteine (C), said mutated RNA polymerases optionally comprising at least one of the following additional mutations: replacement of the tyrosine (Y) in position 640 by a phenylalanine (F), replacement of the isoleucine (I in position 811 by an asparagine (N), those derived from the wild-type KII RNA polymerase, and comprising at least one of the following mutations: replacement of the aspartic acid (D) in position 167 by asparagine (N), replacement of the proline (P) in position 289 by a leucine (L), replacement of the arginine (R) in position 314 by a cysteine (C), said mutated RNA polymerases optionally comprising at least one of the following additional mutations: replacement of the tyrosine (Y) in position 662 by a phenylalanine (F), replacement of the isoleucine (I) in position 833 by an asparagine (N), and those derived from the wild-type SP6 RNA polymerase, and comprising at least one of the following mutations: replacement of the aspartic acid (D) in position 117 by asparagine (N), replacement of the proline (P) in position 239 by a leucine (L), said mutated RNA polymerases optionally comprising at least one of the following additional mutations: replacement of the tyrosine (Y) in position 631 by a phenylalanine (F), replacement of the isoleucine (I) in position 804 by an asparagine (N).
 3. A method for producing a RNA of interest, or proteins coded by this RNA of interest, comprising: synthesizing RNA with mutated RNA polymerases of phage origin, the peptide chain of which is modified, with respect to the wild-type RNA polymerases from which they originate, by substitution, or deletion, or addition of at least one amino acid, this modification having the effect of reducing the sensitivity of said RNA polymerases to the ITS contained in a DNA sequence coding for an RNA of interest, starting from determined nucleotide sequences comprising a DNA sequence coding for said RNA of interest and the transcription of which is placed under the control of a promoter recognized by the abovementioned wild-type RNA polymerases and mutated RNA polymerases, said method having a production yield of said RNA greater than the yield obtained in the case of use of the wild-type RNA polymerases in the presence of the same non-consensual ITS as that contained in the DNA sequence coding for said RNA of interest, wherein said mutated RNA polymerases are selected from the group consisting of: the mutated T7 RNA polymerase represented by SEQ ID NO: 2, comprising a leucine in position 266 substituted for the proline, the mutated T7 RNA polymerase represented by SEQ ID NO: 4, comprising a valine in position 117 substituted for the isoleucine, and an alanine in position 134 substituted for the valine, the mutated T7 RNA polymerase represented by SEQ ID NO: 6, comprising a valine in position 119 substituted for the isoleucine, and an asparagine in position 147 substituted for the aspartic acid, the mutated T7 RNA polymerase represented by SEQ ID NO: 8, comprising an arginine in position 230 substituted for the histidine, and a cysteine in position 291 substituted for the arginine, the mutated T7 RNA polymerase represented by SEQ ID NO: 10, comprising a leucine in position 266 substituted for the proline, and a phenylalanine in position 639 substituted for the tyrosine, the mutated T7 RNA polymerase represented by SEQ ID NO: 12, comprising an asparagine in position 810 substituted for the isoleucine, the mutated T7 RNA polymerase represented by SEQ ID NO: 14, comprising a leucine in position 266 substituted for the proline, and an asparagine in position 810 substituted for the isoleucine, and the mutated T7 RNA polymerase represented by SEQ ID NO: 16, comprising a valine in position 119 substituted for the isoleucine, an asparagine in position 147 substituted for the aspartic acid, and an asparagine in position 810 substituted for the isoleucine. 